Cardiopulmonary diseases afflict millions of people each year. In particular, diseases of the heart remain the leading cause of death in the United States. Monitoring patients' physiological state is an important aspect in the diagnosis, management and treatment of various diseases and disorders, including cardiopulmonary diseases. For this reason, significant efforts have been directed at improving monitoring and detection technologies. In specific, significant efforts have been directed at improving monitoring and detection technologies for cardiopulmonary diseases and related diseases that affect cardiopulmonary parameters.
Implantable medical devices can be advantageous as monitoring devices because the monitoring can be performed as desired, without regard to the physical location of the patient. In addition, the use of implantable medical devices for patient monitoring eliminates problems associated with patient compliance. However, many existing techniques for monitoring patients' physiological state fail to adequately assess all of the parameters desired. For example, cardiac output and related measures such as stroke volume are clinically important but difficult to accurately assess with current implantable systems.
For at least these reasons, a need remains for implantable devices and methods for measuring cardiac performance.